Light and Electron Microscopic Studies on the Postnatal Development of Skeletal Muscle Fibers; in Rats

Abstract

The present study was designed to investigate the morphological changes of the skeletal muscle fibers during the postnatal development. Thirty male albino rats were used for this study. Five animals were sacrificed at each age of 1, 10, 30, 60, 120 and 200 days after birth, respectively. Their M. soleus was fixed in glutaraldehyde and osmium tetroxide, and examined with light and electron microscopes. The results were summarized as follows: 1. Wet weight of rat soleus muscle increased rapidly until 120th day; further increase was less marked thereafter. Muscle weight per body weight (mg/100g) showed 25.7±3.6, 38.1±3.1 at 10th and 60th day, respectively, and thereafter the rate remained unchanged. 2. From 30th to 120th days there found a considerable increase in the diameter and cross sectional area of muscle fibers. The rate of increase were 6.6 and 43.2 fold, respectively, at the stage of 120th day as compared with newborn stage. 3. As regards relative volumes of the intracellular components of soleus muscle fibers, sarcoplasmic volume was, at birth, greater than myofibrillar volume. But the ratio of the myofibrillar volume to the muscle fibers showed an increasing tendency during the postnatal development (i. e. 30, 48, 80 and 86 percent at the stage of 1, 10, 30 and 60 days after birth, respectively). The increase of myofibrillar volume in the proportion to the muscle fibers was especially marked during the first 30 days. These observations suggest that there are two optimum stages of muscular training in rats, namely : 30th-60th days; stage of abrupt development, and 60th-120th days after birth; stable stage of adult.